1. Field of the Invention
The present invention relates to an equalization device, a battery system and an electric vehicle including the same, an equalization processing program, and an equalization processing method.
2. Description of the Background Art
Chargeable and dischargeable battery modules are used as driving sources of movable objects such as electric automobiles. Such battery modules each have a plurality of battery cells (electric cells) connected in series, for example.
There have recently been developed battery modules using lithium-ion batteries as the plurality of battery cells. The characteristics of the lithium-ion battery are more prone than a nickel metal hydride battery to deterioration to be caused by overcharge and overdischarge.
Charge-discharge characteristics vary among the plurality of battery cells. Therefore, it is preferable that charge-discharge of each battery cell is individually controlled in order to prevent the cell from being overcharged or overdischarged.
There has been proposed a method of detecting remaining capacities of the plurality of battery cells (amounts of charges stored in the battery cells in a given state) and equalizing the remaining capacities of the battery cells based on the detected remaining capacities (see JP 2003-284253 A, for example).
A battery module described in JP 2003-284253 A includes a battery block (battery pack), a voltage measuring circuit, a microcomputer, a plurality of bypass resistors and a plurality of field effect transistors (FETs).
The battery block is composed of a plurality of battery cells connected in series. The bypass resistor and the FET are connected in parallel to each battery cell. The voltage measuring circuit is connected to the plurality of battery cells, and acquires open circuit voltage values of the plurality of battery cells. The microcomputer is connected to the voltage measuring circuit and the plurality of FETs.
In the battery module, an adjustment amount and an adjustment time period of the remaining capacities of the battery cells are calculated based on the open circuit voltage values of the plurality of battery cells acquired by the voltage measuring circuit for equalizing the remaining capacities of the plurality of battery cells. At this time, an identification number and the adjustment time period of each battery cell that requires the adjustment (an adjustment target battery cell) of the plurality of battery cells are stored in the microcomputer.
The FETs connected to the adjustment target battery cells are simultaneously switched from an OFF state to an ON state at a given timing. The FETs connected in parallel to the adjustment target battery cells are maintained in an ON state for the calculated adjustment time period, thereby causing currents to flow through the bypass resistors. This causes the adjustment target battery cells to be discharged, thus equalizing the remaining capacities of the plurality of battery cells.
It is difficult, however, to accurately set the remaining capacities of the plurality of battery cells within an appropriate range in the battery module of JP 2003-284253 A. Therefore, each battery cell cannot be sufficiently prevented from being overcharged or overdischarged.